Solar cells have recently gained widespread popularity and the competition among manufactures and products has intensified. As for the composition of solar cells, a large number of types including amorphous silicon, thin-film silicon, and organic compounds have been developed in addition to the conventional single crystal silicon. Accordingly, in order to evaluate fairly the photoelectric conversion efficiency of solar cells, the evaluation methods therefor have been defined by IEC60794 and JIS standard (C8905-C8991). Summarizing the standard, the performance of a solar cell is evaluated by measuring the generated power of the solar cell by using an illumination device (referred to hereinbelow as “solar simulator”) emitting light that simulates a spectrum and an irradiance identical to those of solar light of Air Mass 1.5 (referred to hereinbelow as “reference solar light”). FIG. 6 shows spectrum and irradiance (referred to hereinbelow as “spectral irradiance S(λ)”) of the reference solar light.
As shown in FIG. 7, the solar light of Air Mass 1.5 is obtained when light (AM0) from the outer space falls on the ground surface at an angle of 42 degrees. The AM0 light undergoes scattering and absorption when passing through the air, and spectrum, wavelength distribution, and irradiance thereof change as shown in the below-described FIG. 4.
The generated power can be measured by measuring the values of current and voltage outputted in the DC mode and therefore a predetermined measurement accuracy can be obtained. However, the simulated solar light necessary for measuring the generated power is difficult to generate, and specifications required for the aforementioned solar simulator are defined in C8912 in the aforementioned JIS standard. The principal requirements are described below.                Irradiance should have a predetermined value (1,000±50 W/m2).        Illumination unevenness should be less than a predetermined value.        Spectral irradiance should be less than a predetermined value.        Fluctuations of irradiance with time should be less than a predetermined value.        Illumination light should be parallel light.        
Accordingly, Patent Document 1 suggests a solar simulator configured so that light beams from a plurality of light sources (a xenon light source and a halogen light source) generating light with mutually different wavelength ranges are selectively transmitted/reflected by a mirror having a wavelength dependence and the transmitted/reflected beams are synthesized, thereby generating light having a spectrum similar to that of solar light from UV to IR.
Further, in a product YSS-50A manufactured by Yamashita Denso Co., Ltd., a relative illumination sensor is incorporated in a solar simulator, and the generated power that depends on the fluctuations in the light source is calibrated on the basis of the sensor detection results.
Further, in the configuration described in Patent Document 2, in order to correct light quantity fluctuations of a solar simulator, the solar source irradiance is measured and the response characteristic of the illumination measuring sensor is matched with the response characteristic of the solar cell itself, thereby canceling the light quantity fluctuations of the solar simulator.
The related art suggested in the aforementioned Patent Document 1 and Patent Document 2 represents calibration methods performed with a single solar simulator. However, solar simulators differ among the manufacturers and the devices produced by the same manufacturer can differ from one another, and even if each device satisfies the above-described characteristics, the generated power will differ if the measurements are conducted with different solar simulators.
Accordingly, the measurer sends sample solar cells for measurements, for example, to the National Institute of Advanced Industrial Science and Technology (a national entity having reference solar light spectra that are internationally unified, or an agency corresponding thereto). The agency measures the generated power of the sample by using the proprietary solar simulator that is very close to the reference solar light, describes the measured value (=A) and sends it to the measurer. Upon receiving the measured value, the measurer thereafter uses the returned sample as a reference cell in its own company for calibrating the solar simulators. Thus, the measurer adjusts the light quantity of the solar simulator by using the reference cell so that the generated power becomes the aforementioned measured value A and then measures the generated power of the solar cell that is to be actually measured. With such a method, it is difficult to reproduce strictly the reference solar light spectrum, but this method serves to match solar simulators of each company therewith.
However, with the above-described method, it is necessary for the measurer to produce and send by mail a sample and for the public agency to measure and send back the sample before the calibration with the reference cell is completed. Therefore, the method is time consuming and costly. Moreover, it is not sufficient to perform the calibration once. Thus, the new reference cell should be produced and the calibration should be repeated each time the spectral sensitivity of the solar cell that is to be measured changes, and the aforementioned time and cost increase dramatically.
Patent Document 1: Japanese Patent Application Publication No. H8-235903.
Patent Document 2: Japanese Patent Application Publication No. 2004-134748.